Superassignment (<<-) and indexing

On Fri, 2 Sep 2005, Spencer Graves wrote:

      Permit a mild protest on the word "appropriate" in this 
context.  The
global assignment operator "<<-" provides, for my tastes, excessive
opportunities for problems.  If I define "x" someplace else and then
call your function, it may change my "x" in ways that generate
considerable wailing and gnashing of teeth.

No, no, no.

The sensible and appropriate uses of <<- involve modifying a variable 
that already exists in the lexical parent environment.  In these cases 
it can't escape and ravage the calling environment.

Certainly using <<- to assign to the calling environment is bogus.  In 
addition to your complaints, it doesn't even work (except from the 
global environment), since <<- searches the lexical stack rather than 
the call stack.

In R, <<- can be used safely to maintain state inside a function or 
shared between a set of functions (as in demo(scoping), or 
demo(tkdensity)). In S-PLUS it is admittedly harder to come up with good 
uses.

    -thomas